1. Field of the Invention
The present invention relates to electromagnetic stirring method for improving the quality of a continuously cast strand (i.e. c.c. strand) obtained in horizontal continuous casting process, and more particularly to electromagnetic stirring method in which the produced amount of equiaxed crystals in a center portion of the strand is increased and microcavity or center segregation is suppressed such that the integral quality thereof can be improved.
2. Description of the Prior Art
Development and practical application of horizontal continuous casting processes have been rapidly advanced, and application of electromagnetic stirring to the horizontal continuous casting process is now being studied for the same purpose as in secondary cooling zone stirring in vertical continuous casting processes such as an ordinary bending type or curved type, i.e. for the purpose of increasing the equiaxed crystal zone or improving center segregation. The quality improving effect of a c.c. strand by means of electromagnetic stirring is classified as being a surface quality improvement and internal quality improvement. The latter is directed to the fact that the top end of the columnar crystals growing from outside is cut by stirring the flow of molten steel such that a large amount of equiaxed crystal nuclei are thereby produced, and solidified structure at center portion is transformed into equiaxed crystals so as to improve microcavity or segregation in the center portion.
Equiaxed crystal nuclei produced by electromagnetic stirring are settled by means of the effect of gravity. In the case of an ordinary continuous casting device of a vertical type or a curved type, the c.c. strand is drawn downwards and therefore equiaxed crystal nuclei are apt to settle in the drawing direction and nearly at the center of a cross-section of the c.c. strand. In the horizontal continuous casting process, however, the c.c. strand is drawn in the horizontal direction and therefore equiaxed crystal nuclei settle so as to be accumulated in a downward direction.
For example, referring to FIG. 1 showing a schematic transverse sectional view of a c.c. strand obtained by stirring of one stage in an ordinary horizontal continuous casting process, equiaxed crystals are accumulated at the lower side during drawing of the strand and the upper side is apt to be occupied by columnar crystals and thus resulting in serious problem occurring from the viewpoint of quality (In FIG. 1, A is a columnar crystal forming zone, B denotes an equiaxed crystal forming zone, and broken line W indicates the depth of solidified shell thickness). In this connection, it is known that development of columnar crystals causes an increase of center segregation. For example, if such c.c. strand is rolled into welding steel material, welding defects will occur at the segregation portion. If the strand is formed into a wire rod, a cuppy fraction is produced and drawing of thin wire cannot be performed. Furthermore, if the strand is used for cold-rolled thin sheet, a fine ridging flaw may occur on the skin of steel sheet surface, most notably in stainless steel. Since the solidified structure is not uniform in the vertical direction of the transverse cross-section, the above-mentioned defect will be deviated to one side of the product.
In order to eliminate above-mentioned disadvantages, a method as set forth in Japanese patent application laid-open No. 57-75258 was proposed. In this method, equiaxed crystal nuclei are transferred towards the crater end using an electromagnetic stirring coil of a linear motor type so as to enlarge the equiaxed crystal forming zone and obtain a uniform solidified structure similar to c.c. strand in vertical continuous casting process. However, this method requires a long coil due to the special condition in the structure of a linear motor type coil, and since uniform spray cooling throughout such a long coil is difficult, a lack of uniformity in the cooling tends to cause surface cracking or deformation in the c.c. strand. Moreover, a coil of linear motor type has a poor stirring efficiency in comparison with that of a rotary magnetic field type, and, in order to attain the stirring efficiency comparable with that of the coil of rotary magnetic field type, a coil of large size must be used. Therefore the cost for equipment increases.